In a single switched forward converter, the core reset is done by a third winding with opposite dot polarity to the primary and secondary windings. This dissipates the magnetizing inductance but how is the leakage inductance dissipated?

If it's left unaccounted for doesn't the remaining energy in the leakage inductance cause huge back EMF? If you use an RCD snubber/clamp the magnetizing energy as well as the leakage energy goes through this clamp which would cause huge power dissipation and inefficiency.

How does a high power (>100 watt) forward converter deal with this leakage inductance without going to a two switch half bridge design?

Thank you


The short answer is to snub it.

For a Forward with reset winding, best is probably a clamping snubber. Here is a schematic showing snubber specific to the Forward converter.

enter image description here

Shown are only the primary and reset windings of the transformer. Basic idea is to put the energy from magnetizing and leakage inductance into C1. When Q1 is on, the reset winding puts negative voltage on C1 equal to Vin. When Q1 turns off, magnetizing and leakage inductance cause Q1 voltage to rise above Vin. Any rise of Q1 voltage above 2 Vin will be clamped by D1 and C1, also resetting the core. When Q1 turns back on, voltage on C1 will normalize to Vin.

C1 needs to be large enough to allow only a small voltage change from the energy transferred from the magnetizing and leakage inductance each cycle. D1 must withstand at least 2 Vin and full spike current. Some resistance might be needed in series with the reset winding to damp resonance between C1 and reset winding.

This usually the lowest loss way to snub a Forward converter with reset winding.

Of course, you could bypass all of this and use a 2 switch forward. That would also allow a more common type of transformer.


I searched for "single switched forward converter" and read what Wikipedia had to say. It seems that flyback converters use inductors, which store all their energy in their leakage inductance. But this circuit uses a transformer, and, "transformers cannot store a significant amount of energy, unlike inductors". So, your concerns are correct, but apparently the effects of leakage inductance will be far smaller than you are expecting.

The remark about transformers indicates that almost all of the energy put into the primary is immediately transferred to the secondary and then to the load.

  • 1
    \$\begingroup\$ Some insight into these kinds of converters can be found in this PDF. It's from 1994, but the magnetics discussion still applies today. See p.116 (PDF p.14). \$\endgroup\$ – Nick Alexeev Jan 3 '15 at 20:24

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